1. Fields of the Invention
The present invention relates to a projection system, especially to a laser projection system in which a laser projector emits laser beams according to an image signal of a static image or a dynamic image and projects the laser to a corresponding projection screen to generate an image. Thus a projection nearly transparent is displayed in a nearly transparent manner on the projection screen in natural sunlight. The projected image and objects behind the projection screen are seen at the same time.
2. Descriptions of Related Art
A lot of designs related to laser projection system are available now. Among them, U.S. Pat. Nos. 6,986,581, 7,090,355, 7,182,467, 7,213,923, 7,452,082, etc. are related to films of projection screens. Refer to U.S. Pat. No. 6,843,568 and the references cited, a laser scanning display is revealed. In the U.S. Pat. No. 6,329,966, techniques related to UV beam on phosphorus dots on film are revealed. The U.S. Pat. No. 4,213,153 discloses images formed by modulated UV laser on phosphorus material. In the U.S. Pat. No. 6,900,916, techniques regarding scanned UV light to generate images on fluoresce film are disclosed.
Refer to FIG. 1, a common laser projection system 100 includes a laser projector 10 and a projection screen 101. According to the image signal S1, the laser projector 10 projects images onto the projection screen 101. The laser projector 10 consists of a laser light source module 110 and a rotation plane mirror module 120. The laser light source module 110 is composed of a visible red laser light source 111, a visible blue laser light source 112, a visible green laser light source 113, an alignment module 115, and a modulation module 114. The visible red laser light source 111, the visible blue laser light source 112, and the visible green laser light source 113 respectively are used to provide a visible red laser beam LR, a visible blue laser beam LB, a visible green laser beam LG according to driving current IRD, IBD, and IGD. In accordance with an image signal S1, the modulation module 114 provides the respective driving current IRD, IBD, and IGD to the visible red laser light source 111, the visible blue laser light source 112, and the visible green laser light source 113 for modulation of the laser beams LR, LB, and LG so as to achieve light intensity of the figures and words required on the projection screen 101. As to the alignment module 115, it aligns the laser beams LR, LB, and LG to generate a modulated laser beam LM. The rotation plane mirror module 120 is for reflecting the modulated laser beam LM to the projection screen 101. By the rotation plane mirror module 120, the laser projector 10 adjusts a deflection angle θ and a deflection angle φ for changing the direction of the modulated laser beam LM after reflection. Thus the modulated laser beam LM scan each position corresponding to each pixel of the projection screen 101 quickly and an image is displayed on the projection screen 101 due to the modulated laser beam LM being scattered by the projection screen 101.
The laser projection technology mentioned above has following disadvantages:
1. For images formed by scattering, the projection screen is not transparent. Thus the applications of the imaging technique to transparent screens are restricted. For example, the transparent screen may be a head-up display that projects images onto a car glass windshield.
2. The deflection angles θ, φ of the rotation plane mirror module are limited. Thus the distance D between the projection screen and the laser projector needs to be increased so as to generate a larger projected image.
3. When visible light of the background light is lighter and emitted to the projection screen, the color contrast of the images formed is reduced.